The first stage of our study was concerned with the immunobiological properties of bone allografts and their usefulness in intrauterine repair of fetal bone defects. Allogeneic bone transplantation in utero in the Macaca mulatta model has been carried out successfully. Two techniques were applied. The first consisted of correcting a surgically created loss of a bone segment of the fetal humerus with bone-plug taken from a different fetus. The second was the application of a bone-paste, consisting of allogeneic bone particles suspended in an agar-base culture medium to a surgically ablated humerus of the host. Both techniques were very successful and provided firm evidence for the permissiveness of the fetal immune surveillance system in the acceptance of allogeneic bone transplants in primates. It was determined that this permissiveness persists for up to eight weeks of postnatal life. The second stage of our study, which is now in progress, involves the diagnosis and in utero surgical correction of skeletal defects, such as hydrocephaly, meningocele, encephalocele and spina bifida. These conditions are being induced at an early stage of gestation by treating monkeys with teratogens.